State of the art receivers for wireless communication (e.g., in mobile terminals or base stations), operated in accordance with the Global System for Mobile communication (GSM), Wideband Code Division Multiple Access (WCDMA), or Long Term Evolution (LTE, including evolved UMTS Terrestrial Radio Access Networks, or E-UTRANs), are equipped with Automatic Gain Control (AGC). The AGC is used to ensure that a signal received by the receiver is processed at a suitable signal level (also referred to a signal power). In this regard, the AGC scales the received signal such that the signal can be represented by a limited number of bits for digital processing without significant loss of information. The receiver gain determined by the AGC should be large enough to minimize quantization noise. At the same time, the AGC should ensure that the scaled signal does not exceed a maximum range of the digital processing, i.e., that the processing in the receiver or in a downstream processing stage does not get saturated.
FIG. 1 schematically shows a received signal 10 interfered by bursts of an intermittent interference 12. In case the intermittent interference 12 is sufficiently distant in frequency space from the signal 10 to be received, the interference 12 might be sufficiently attenuated by means of an analog filter, so that its impact on the digital processing of the received signal 10 is marginal. Otherwise, the AGC has to take into account interference 12 that is close in frequency space, or inside the spectrum of signal reception, even if the interference 12 could be attenuated by the digital processing.
The AGC determines the receiver gain depending on both the signal to be received and the interference 12. In case the received power of the interference 12 is considerably larger than the received power of the signal 10, the receiver gain determined by the AGC essentially depends on the received power of the interference 12. Thus, the receiver gain is significantly reduced from a receiver gain 14, which is appropriate when only the signal 10 is received, to a receiver gain 16 determined by the interference 12.
The gain reduction in a stage of the receiver prior to the digital processing is detrimental to signal quality and increases quantization noise, as is expressed by the Friis formula for noise. As a quantitative example, for a signal amplitude Asignal and an interference amplitude Ainterference, a number of B bits is lost in a dynamic range of the digital processing, wherein Asignal/Ainterference=2−B. This fact is related to a difference in the received power of the signal 10 and the interference 12, which difference is 20·log10 (Ainterference/Asignal) dB=6.02·B dB. In case the interference 12 would be 30 dB stronger than the signal 10, the conventional AGC would shift the dynamical range by B=5 bits in order to prevent saturation. Due to a “conservative” updating of the old receiver gain settings 14 to the new receiver gain setting 16 at time t1, the loss in dynamic range of B=5 bits for the digital processing covers a time interval from time t1 to time t2.
Requirements on power consumption and hardware resources typically limit an available dynamic range of, for example, a mobile terminal to effectively 60 dB or approximately 10 bits. Such a loss in the order of half of the available dynamic range is detrimental to processing the signal 10 received using the reduced receiver gain 16. Consequently, signal reception may be impeded not only during presence of the interference 12, but even beyond in the time interval from time t1 to time t2, as well as the time following t3. As a result, a communication can be temporarily interrupted or may even break down.
For the case of periodic interference described in U.S. Pat. No. 7,263,143 B1, an occurrence of interference events might be predictable. The AGC can be improved under such specific conditions based on the prediction. In a more general random interference scenario, the conventional AGC technique is expected to still suffer from the loss of dynamic range.